This invention relates to a system and method for providing high-speed digital communications through a communications channel, and more particularly to an equalizer for communications systems implemented in wired type environments, for example microstrip, stripline, printed circuit board (e.g., a backplane) and cable.
Communications systems are continuing to increase the rate at which data is transmitted between devices. The increase in data rate presents a challenge to maintain, enhance or optimize the ability to recover the transmitted signal and thereby the information contained therein. Thus, in general, increasing the rate of transfer of data tends to adversely impact the fidelity of the received signal.
For example, high-speed digital baseband communications systems often encounter debilitating signal reflections and signal dispersion as the rate of data transfer increases. Signal reflections are often due to mismatches of impedances between the impedances of the devices (whether the devices are connected in a point-to-point or bus configuration), and/or mismatches of the impedance of the communications channel and termination resistors.
Signal dispersion, also known as intersymbol interference, may be caused by bandwidth limitations of the communications channel. Dispersion in many situations is due to two primary effects, namely, dielectric loss and skin effect. The effects of dielectric loss are often limited or minimized in communications systems, for example, wired systems, through careful design of the conductor insulator.
In backplane environments, however, intersymbol interference caused by dielectric loss may be difficult to eliminate, or sufficiently or adequately limit or minimize. In this regard, dispersion of the transmitted signal may be observed in the time domain as a symmetric broadening of that signal. This broadening of the transmitted signal produces both pre-cursor (before the pulse peak of the signal) and post-cursor (after the pulse peak of the signal) intersymbol interference.
Dispersion effects attributable to skin-effect tend to be observed predominately as post-cursor intersymbol interference. Although the debilitating effects caused by skin-effect may not be dominant when compared to effects of dielectric loss, skin-effect may be a source of dispersion to be addressed at higher transmission rates.
In short, pre-cursor and/or post-cursor impairment, whether due to skin effect or dielectric loss, must be addressed as transmission rates increase.
With reference to FIGS. 1 and 2, conventional high speed digital baseband communications systems often employ circuitry, for example, a finite impulse response filter (“FIR filter”), in the receiver to equalize the transmitted signals in an effort to address, or compensate for the effects of dispersion and reflection of the transmitted signal and/or the sensitivity of the system to that distortion or reflection. Such circuitry typically includes a one or more taps having fixed or pre-programmed “positions” and coefficients. These taps are typically “trailer” or “trailing” taps and, as such, the equalization circuitry address only post-cursor signal distortion.
In pre-emphasis equalization implementations (i.e., equalization circuitry and techniques implemented in the transmitter), the equalization circuitry also includes “trailer” or “trailing” taps to provide an equalization signal that is produced after transmission of the information signal. The pre-emphasis equalization circuitry, like the equalization circuitry implemented in the receiver, is designed to address only post-cursor signal distortion.
For example, in the backplane environment, conventional communications systems employ an FIR filter having fixed or pre-programmed tap positions and coefficients in the transmitter. The taps are positioned to compensate for post cursor intersymbol interference.
Regardless of where the equalization circuitry is implemented, the duration of the equalization signal and the relative position or placement of the tap(s) of the equalization circuitry are selected or designed to avoid interference with the signal representative of the transmitted information. As such, at the transmitter, there is no temporal overlap between the equalization signal and the information signal. In this way, the equalization signal is less likely to interfere with the pulse peak of the transmitted signal (i.e., the symbol or data signal).
Thus, while conventional equalization techniques may address, or compensate for the effects of some of the bandwidth limitations and reflections in the system, the ability of such techniques to provide sufficient compensation for high-speed communications may be limited. Moreover, not only may the conventional techniques be unsuitable to provide adequate compensation for the debilitating affects on the integrity of the transmitted signal in high speed communication systems, but the operation and corresponding impact of conventional equalization circuitry may not be adjusted as the environment of those systems change (for example, due to changes in temperature, operating conditions, data rate, and device parametrics due to, for example, aging). That is, after design and manufacture, conventional equalization circuitry and techniques have limited flexibility when implemented within a particular environment or an environment that varies over time. This may severely limit the usefulness of such equalization circuitry and techniques when implemented in environments that change dramatically over time.
Notably, incorporating more complex equalization circuitry, for example, an FIR filter having many taps, tends to add cost, complexity and power consumption to a transmitter, receiver and/or transceiver. In addition, conventional pre-emphasis tends to suffer from over-equalization at the boundaries of the symbol (data signal) and may exhibit large parasitic capacitances thereby degrading the performance of the system. Such over-equalization may impact successive symbols or data signals, thereby contributing to the debilitating effects of signal reflections and dispersion. Accordingly, there is a tendency to implement equalization circuitry and techniques having a minimum of complexity and taps; however, such circuitry and techniques often are unable to provide sufficient compensation for high-speed communications systems.
Thus, there is a need for improved digital communications systems and techniques in order to enhance the performance of, for example, high-speed digital communication systems through a communications channel, for example a backplane. There is a need for improved equalization circuitry and techniques that are capable of compensating for bandwidth limitations and reflections, improving the signal integrity in high-speed communications, and overcoming many of the shortcomings of conventional circuitry and techniques.